The National Social Life, Health, and Aging Project (NSHAP) is a longitudinal study which extends our understanding of the mechanisms through which the trajectories of social connectivity and health are intertwined in an aging population. The project is unique among nationally representative studies of older adults in the breadth and detail of health data collected, incorporating physical, psychological, sensory, functional, cognitive and social measures. In particular, we have unparalleled longitudinal measures of changes in the structure of social networks and relationships of respondents and their partners throughout the study as well as information on Alzheimer's disease and related dementias (ADRDs). We propose to conduct in-home interviews in 2020?21 (Wave 4 [W4]) with all surviving respondents and their spouses/partners from the first three waves (W1-W3), providing 15 years of follow-up and four waves of data on this important cohort (aged 72-100 at W4). For innovations introduced in W2? a multi-domain measure of cognition (the MoCA-SA), detailed olfactory assessment, complete data from spouses/partners, an accelerometry study of sleep and daily activity, and more comprehensive measures in all domains?W4 will provide a third data point needed to model changes in health, associations, and causal relationships between them. We will enhance the data with new measures of cognition, including item response times, proxy reports, and diagnosis of ADRDs; linking to administrative data files, such as Medicare; and objectively measuring, concurrently, all five classic senses. As one example, W4 will allow the testing of key hypotheses regarding cognitive changes from normal function through dementia, providing important insights into Alzheimer's disease. With these data, we aim to extend our understanding of the mechanisms through which the trajectories of social connectivity and health are intertwined in an aging population. Additionally, we will develop a framework for enrolling the oldest NSHAP subjects into internal and external investigator initiated studies addressing novel social, behavioral, and clinical questions (such as neurodegenerative diseases like Alzheimer's disease and related dementias) in representative samples of this historically understudied population. With its rich and novel measures of social networks and of the biological, physical and psychological components of health?all measured longitudinally from both partners in a couple?NSHAP is uniquely poised to bridge the gap between large, epidemiological studies of social relationships and health and experimental studies of the mechanisms involved.